Think of concentric circles
They represent equipotential surfaces of a point charge placed at the centre (in 2D neglecting the 3rd dimension)
Now, orthogonal trajectory is perp. to all of these circles
Also electric field is perpendicular to potential surfaces.
Now electric field of a point charge is straight lines passing thru the charge
So orthogonal trajectory of concentric circles is the system of straight lines passing thru the centre (in this case origin)
So, the orthogonal trajectory is y=mx